A microporous polyolefin film is broadly used as various battery separators, separation filters, separation membranes for microfiltration and the like due to its chemical stability and excellent physical properties. A wet method of methods for preparing a microporous film from polyolefin is a method of making pores on the polyolefin by kneading the polyolefin with a diluent at high temperature to form a single phase, phase separating the polyolefin and the diluent during a cooling process, followed by extracting the diluent portion. It can produce a thin film with uniform thickness, and has excellent physical properties by stretching, thereby being broadly used as a separation membrane for a lithium ion secondary battery.
Methods for preparing a porous film by the wet method are classified into a solid-liquid phase separation method and a liquid-liquid phase separation method depending on that a polymer making up a film and the kneaded diluent are phase separated through which process to form pores. Both methods are identical up to the step of making a single phase by mixing the polymer and the diluent at high temperature, but different on phase separation mechanism. Accordingly, characteristics of the microporous films finally produced are different each other.
In the case of solid-liquid phase separation, the polymer is crystallized through cooling, and only the polymer is solidified. Accordingly, phase separation occurs as the diluent is removed between the polymer solid phases. In other words, the phase separation occurs as the diluent is pushed out of crystals during crystallization of polymer chains. Therefore, size of the generated phase-separated phase is very small, compared to the size of the polymer crystal, and there is a defect that the structure of the separated phase such as shape and size can't be controlled variously. In this case, there may be a limit to be applied as a secondary battery separation membrane having high transmittance required for a high performance, high output and long life time secondary battery, which is recently being developed at secondary battery manufacturers. As representative composition of the solid-liquid phase separation, a case mixing paraffin oil, mineral oil and paraffin wax to a polyolefin resin is widely known.
In the case of the liquid-liquid phase separation, a polymer in liquid state and a diluent in other liquid phase (partially immiscible) exist as a uniform single phase at the temperature over the liquid-liquid phase separation temperature, and after lowering the temperature, are phase separated by thermodynamic instability at the temperature over the polymer crystallizing temperature. Accordingly, shape, size and the like of the separated phase (droplet) are changed as the phase separation condition is changed. Thus, in the case of the liquid-liquid phase separation method, the temperature of the liquid-liquid phase separation and the size of the separated phase can be controlled according to a kind of a polymer (e.g.,: polymer molecular weight) and a combination of a diluent (e.g.,: solubility parameter), and also there may be the time for growth of the droplet by the liquid-liquid phase separation as much as the difference between the thermodynamic liquid-liquid phase separation temperature (cloud point) and the solid-liquid phase separation temperature. When manufacturing a membrane, a fast freezing process is conducted between a T-die of an extruding machine and a casting roll. Accordingly, the droplet size can be variously controlled depending on the temperature difference between the real temperatures conducting liquid-liquid phase separation and solid-liquid phase separation, and the retention time at the extruding step. In the case of the microporous film prepared by the liquid-liquid phase separation method, the pore size can be controlled unlike the microporous film by the solid-liquid phase separation method, and also a microporous film having the pore size several times larger than the pore size of the microporous film by the solid-liquid phase separation method can be prepared.
On the other hand, in order to use the liquid-liquid phase separation method, a diluent having the liquid-liquid phase separation condition (temperature), which is suitable to polymer processing conditions at the higher processing temperature, should be found.
U.S. Pat. No. 4,247,498 disclosed various combination of a polymer and a diluent, which can be liquid-liquid phase separated, and described that a product having a wide range of thickness can be produced by extracting the diluent from the liquid-liquid phase separated composition. U.S. Pat. No. 4,867,881 described an invention making a microporous film, which is oriented by stretching, extracting, drying and heat-setting a liquid-liquid phase separated composition.
Korean Patent No. KR20080055061A also described the combination ratio of a polymer and a diluent and the combination ratio of two diluent ingredients when processing and extruding.
Therein, kinds of the diluent, which can be used for preparation by the liquid-liquid phase separation were listed, and those were selected from the existing diluents suitable for liquid-liquid phase separation, listed in U.S. Pat. No. 4,247,498, based on manufacturing characteristics.
However, still it is needed to develop a microporous polyolefin film, wherein the pore size is effectively controlled by using a diluent having excellent liquid-liquid phase separation characteristics with polyolefin.